Patients that have respiratory difficulties often must be placed on a mechanical ventilator. These difficulties may be pathological in nature or may be due to the fact that a patient is too weak or sedated to independently perform proper respiration functions. Often, the patient may be spontaneously attempting to breath, but not able to complete a full respiratory cycle. In these cases, mechanically assisted ventilation is provided. In mechanically assisted ventilation, a combination of pressure and/or flow sensors detects a patient's breath attempt. This detection triggers the delivery of a mechanical breath, which is provided in the inspiratory phase by the delivery of a pulse or plug of medical gases under a pressure that is sufficient to overcome the resistance of the patient's airway, thus filling the lungs. When this pulse of medical gas is discontinued, the natural compliance of the patient's chest wall forces the delivered breath out of the patient in an expiratory phase.
Often, mechanical ventilation is supplemented by an additional delivery of medical gas, such as oxygen or nitric oxide, to the patient. This additional gas may be supplied within the mixture of medical gases delivered during mechanical ventilation, or the supplemental oxygen may be delivered to the patient directly through the nostrils by the use of a nasal cannula. Additionally, a patient who is not on mechanical ventilation, but rather is spontaneously performing complete respiration cycles, may receive supplemental oxygen via a nasal cannula to increase the oxygen uptake by the lungs. In situations where a patient is receiving supplemental oxygen and/or mechanical ventilation, it is desirable to measure the end tidal carbon dioxide in the patient's exhaled breathing gases. This is a useful medical quantity as it is indicative of the patient's respiratory efficiency as well as a useful diagnostic tool for an attending clinician. The monitoring of end tidal CO2 levels can provide the clinician with information regarding oxygen-carbon dioxide exchange, alveolar recruitment, and acid-base disorders.
In prior systems that utilize a non-invasive ventilation (NIV) mask for patient ventilation and also monitor CO2 levels in the expired patient gases, leaks within the system cause diluting effects, thereby making CO2 measurements at the Y-piece or in the expiratory limb difficult.
The efficiency of the mechanical ventilation of a patient may be increased by performing accurate triggering of the delivery of ventilator support in association with a patient's spontaneous breath attempt. Known ventilators and breathing circuits comprise a variety of flow and pressure sensors that produce signals to detect breathing effort by the patient and may trigger the ventilator to deliver a breath to the patient that is synchronous with those efforts. In known arrangements, the flow and/or pressure sensors are placed in the patient breathing circuit, in the patient breathing circuit interface, or in the ventilator.
One common method of mechanically ventilating a patient includes a non-invasive ventilation (NIV) breathing mask applied over the nose and mouth of a patient to deliver the ventilation gases to the patient. However, in this type of an arrangement, if the ventilator is delivering positive air pressure and the NIV mask is inadvertently pushed against the patient, the patient sensing mechanism will identify the resulting increase in pressure and interpret the increase as a patient's attempt to cycle the breath to expiration. This false identification would be due to the increased pressure in the face mask, resulting from the inadvertent compression of the face mask.
In another event, if a circuit leak occurs during the expiatory phase of the breath, the sensing mechanism would identify the resulting pressure change and interpret it as a patient's attempt to trigger a breath. Such a misinterpretation results in asynchrony between the ventilator and the patient's respiratory efforts, ultimately reducing the assistance provided to the patient. While leaks can occur anywhere in the circuit, a common location for leaks is between the patient/breathing circuit interface and the patient, which is typically where the face mask meets the patient's face.
Therefore, it is desirable in the field of medical gas delivery to patients to provide a nasal cannula and control system that provides effective patient triggering when used in conjunction with a mechanical ventilator. Further, it is desirable to provide a nasal cannula control system that is able to monitor the amount of a selected gas in the expiratory gas flow from the patient and provide a signal indicative of the sensed gas concentration.